1. Field of the Invention
The present invention relates to a water base ink for ink-jet recording to be used for an ink-jet recording apparatus.
2. Description of the Related Art
In the ink-jet recording system, ink droplets are formed by using the ink discharge system including, for example, the electrostatic attraction method, the method in which mechanical vibration or displacement is applied to the ink by using a piezoelectric element, and the method in which bubbles are generated by heating the ink to utilize the pressure generated thereby. All or a part of the ink droplets are adhered to a recording objective material such as paper to perform the recording. Those usable as the water base ink for ink-jet recording to be used for the ink-jet recording system as described above include those which are obtained by dissolving or dispersing a variety of water-soluble dyes or pigments in water or a liquid medium composed of water and a water-soluble organic solvent.
In order to perform the satisfactory recording over a long period of time, for example, the following conditions are required for the water base ink for ink-jet recording as described above. That is, characteristic values including, for example, those of the viscosity, the surface tension, and the density are appropriate values; any deposit is not formed and/or physical values are not changed, for example, by the heat in order to avoid any clog-up at the nozzle and the orifice of the ink-jet recording apparatus so that the ink is discharged in a stable manner; and the recorded image is excellent, for example, in water resistance and light resistance.
When the recording is performed with an ink-jet printer by using a general water base ink for ink-jet recording, any exclusive ink-jet paper is sometimes used in order to obtain a satisfactory printing quality without any blurring. However, in recent years, it is more demanded that the recording is performed on the regular paper rather than on the exclusive ink-jet paper, in consideration of the running cost and the environment. Further, in the market directed to homes and offices, it is overwhelmingly demanded that the recording is performed with the colors rather than with the black-and-white or monochrome. Therefore, the color ink-jet printer is dominantly used. It is demanded that the color recording can be performed with a good printing quality on the regular paper.
However, the printing quality on the regular paper is still insufficient. Major factors therefor may include various factors. One of such factors is the problem called “feathering”. In this case, the ink is nonuniformly blurred along the paper fibers of the recording paper when the ink is permeated into the recording paper. The edges of image portions are notched, and it is impossible to obtain any sharp edge of the image portion.
In relation to such circumstances, a method, in which the surface tension is increased to obtain an ink composition that scarcely causes the blurring or spread into the paper fibers of the recording paper, has been hitherto widely known as a general technique for improving the printing quality by avoiding the feathering. For example, Japanese Patent Application Laid-open No. 8-259864 discloses a technique in which the surface tension of an ink is made to be not less than 40 mN/m so that the permeation of the ink along the recording paper surface is suppressed and the feathering is avoided. However, in the case of this method, the wettability of the ink with respect to the paper is deteriorated, and ink portions, which are adjacent to one another on the recording paper, are not continued to give a discontinuous state, resulting in such a printing quality that the spread of the ink is insufficient. In order to avoid such an inconvenience, it is also conceived that an extremely large amount of the ink is discharged. However, in this case, new problems arise, for example, such that the cost is unsatisfactory and the ink is dried over an excessively long period of time. Therefore, an ink is required, which is spread appropriately and uniformly with an appropriate amount of droplets.
On the other hand, a method has been widely used as a general technique for enhancing the permeability of the water base ink for ink-jet recording, in which mutually adjoining ink droplets are continued to one another on the recording paper to improve the printing quality by blending, as a permeating agent, alkyl ether of polyvalent alcohol such as diethylene glycol monobutyl ether to an ink, or by blending a surfactant thereto. For example, Japanese Patent Application Laid-open No. 8-283631 discloses a technique in which a specified permeating agent and a surfactant are blended to an ink to lower the surface tension so that the permeability into the paper is enhanced. However, in the case of this method, it is impossible to suppress the phenomenon in which the ink is nonuniformly spread along the paper fibers. As a result, the edges of image portions are blurred, and the feathering tends to occur.
As described above, the conventional water base ink for ink-jet recording has involved such a problem that it is difficult to satisfy both of the prevention of the feathering and the prevention of the insufficient ink spread on the regular paper.
Further, another problem arises such that the ink is not discharged upon the start of use if the water base ink for ink-jet recording is not introduced smoothly into an ink flow passage of a printer head when a new ink cartridge is installed to an ink-jet printer. Therefore, it is also required for the water base ink for ink-jet recording that the initial introduction performance of the ink into the printer head is excellent in addition to the requirements for the prevention of the feathering and the prevention of the insufficient spread of the ink as described above.
However, in recent years, the miniaturization is advanced for the printer head for the ink-jet printer. For example, a stacked or laminated type printer head is known as a printer head for a piezoelectric ink-jet printer based on the use of piezoelectric elements, which is formed by stacking a plurality of sheets formed with parts of ink flow passages. In the case of the stacked type printer head, the miniaturization is realized at the low cost, and the ink flow passage, which is narrow and which has a complicated structure, can be formed therein. However, bubbles or foams tend to remain, for example, at bent sections 15a, 15b formed on the inner circumferential surfaces of ink flow passages 14a, 14b, 14c and in gaps formed between stacked sheets 10a, 10b, 10c as shown in FIG. 2. It has been impossible to smoothly introduce the water base ink for ink-jet recording into the ink flow passages, and it has been especially difficult to satisfy the initial introduction performance of the ink. FIG. 2 shows a partial magnified sectional view illustrating the stacked type printer head.
In view of the above, a method is known as a method for improving the initial ink introduction performance of the water base ink for ink-jet recording, in which an appropriate amount of a surfactant is added to the water base ink for ink-jet recording to lower the surface tension down to an optimum value in order to improve the wettability with respect to the inner wall of the ink flow passage of the printer head.
However, in the case of the method in which the surfactant is added to the water base ink for ink-jet recording to lower the surface tension so that the initial introduction performance of the water base ink for ink-jet recording into the ink flow passage is improved, the following problem arises. That is, the wettability with respect to the recording paper is increased simultaneously with the decrease in the surface tension of the ink, and the edges of image portions are blurred to cause the feathering. In other words, the conventional water base ink for ink-jet recording has involved such a problem that it is difficult to satisfy both of the initial introduction performance of the ink into the printer head (especially into the stacked type printer head) and the prevention of the feathering.
As described above, when the recording is performed on the regular paper by using the conventional technique, the following problem has arisen. That is, all of or any one of the troubles arise, for example, such that the feathering is caused on the recording paper, the ink is insufficiently spread on the recording paper, and any trouble occurs in relation to the initial introduction performance of the ink into the printer head. As a result, it has been impossible to obtain any one which has the satisfactory printing quality.